Type 1 Diabetes (T1D) is an autoimmune disorder whereby insulin-producing beta cells are destroyed by the immune system. Diabetogenic CD8+ T cells that recognize islet specific antigens home to the pancreas and initiate an attack on these beta cells. This leads to devastating chronic hyperglycemia and occasional, and life threatening, hypoglycemia. Genetic studies have identified several susceptibility loci that are predictive of T1D, and careful monitoring in the clinic can identify at risk patients who are in a pre-diabetic state. However, despite this knowledge, there exists no reliable method to prevent the initial autoimmune attack on the beta cells or future attacks against transplanted ones. The goal of the work proposed here is to develop a tolerogenic vaccine that is able to interfere with this process and induce regulatory T cells (Tregs) that are protective of islet specific antigens. To do so, this project is aimed at exogenously expressing these antigens in resting dendritic cells (DCs), which are capable of directing a pro-tolerogenic response in the absence of maturation or pro-inflammatory stimuli. To that end, a key challenge is to be able to restrict antigen expression from DCs when they become activated, but maintain robust expression in ones that have not. Our laboratory has recently developed a new microRNA (miRNA) based targeting strategy that is capable of doing just that. By placing target sites for miRNAs that become active upon DC maturation into a transgene, it is possible to completely eliminate expression in activated cells. Therefore, I plan to use viral vectors to deliver islet-specific antigens (Proinsulin and GAD65) to non-activated DCs, and restrict their expression in activated ones by incorporating target sites for miR-155, a miRNA that becomes highly active in mature DCs. I will also use a high-throughput method to identify new miRNAs that can achieve this same goal. These constructs will then be tested to see if they can induce tolerance to Proinsulin or GAD65 and prevent diabetes onset in a NOD mouse model of the disease.